A lubricant is a substance introduced between two moving surfaces to reduce the friction and wear between them. Lubrication occurs when the opposing surfaces are separated by the lubricant (typically a fluid). In general, four regimes of lubrication are broadly defined based upon the mechanism by which the lubricant operates to reduce friction and wear between the moving parts. They are hydrodynamic regime (where a thick film of fluid separates the moving surfaces), mixed regime (where a thin film separates the moving surfaces), boundary regime (where most of the lubricant is squeezed out from between the moving parts), and enhanced pressure regime (where substantially all the liquid is squeezed out from between the moving parts and a thin solid film is formed on the surface of the moving parts). Lubricants are typically made by blending a base oil (most often petroleum fractions) with any number of additives. The additives impart special properties, such as reduced friction, reduced wear, increased viscosity, improved viscosity index, resistance to corrosion, oxidation, aging, and/or contamination, etc. to the lubricant. The functional group contained in the most commonly used anti-wear and anti-friction additives are boron (B), copper (Cu), phosphorous (P), sulfur (S), nitrogen (N), lead (Pb), and/or zinc (Zn).
Many of the lubricants and some additives currently being used are made of petroleum products that are toxic, making it increasingly difficult for safe and easy disposal. There has been an increasing demand for environmentally safe lubricants in recent years due to concerns regarding accidental spillage or leakage of the lubricants and increasingly strict government regulations restricting their use.
U.S. Patent Publication 2006/0009365 A1 issued to Erhan et al. (hereinafter the '365 publication) describes a sulfur modified vegetable oil that can be used as an additive for a lubricant. In the '365 publication, the lubricant additives are created by reacting epoxidized triglyceride oils (vegetable oil) with thiols (having the general formula HS—R′″). The resulting sulfur containing poly (hydroxy thioether) derivatives are environmentally safe because they are formed by modifying a vegetable oil.
Although the lubricant additive of the '365 publication may be environmentally safe, it may have some performance limitations. The sulfur containing additives of the '365 publication have the structural formula:
where Rs, Rs′, Rs″ are characterized by the formula:
where R″ is hydrogen, a C1 to C22 hydrocarbon, 4-6 member heterocyclic ring, or a mixture thereof.
The additive of the '365 publication is restricted to sulfur as the functional group. Therefore, the additive does not offer flexibility in designing an additive with a different functional group which may be more suited to an application. For instance, the additive cannot be designed with phosphorous or an amine as the functional group to suit a particular application. In addition, the presence of the thio-ether group (C—SR′″) and the hydroxyl group (C—OH) in the additive of the '365 publication (see Formula 2) leads to a higher viscosity because of the inter and intra molecular hydrogen bonding within the thio-ether molecules.
The present disclosure is directed at overcoming one or more of the shortcomings of the prior art anti-friction and anti-wear lubricant additives.